From: Candle Sun <[email protected]> Though do_overwritten() follows do_nothing() in source code, the final memory address order is determined by compiler. We can't always assume address of do_overwritten() is bigger than do_nothing(). At least the Clang we are using places do_overwritten() before do_nothing() in the object. This causes the copy size in lkdtm_WRITE_KERN() is *really* big and WRITE_KERN test on ARM32 arch will fail.
Compare the address order before doing the subtraction. Signed-off-by: Candle Sun <[email protected]> --- drivers/misc/lkdtm/perms.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-) diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c index 2dede2ef658f..fbfbdf89d668 100644 --- a/drivers/misc/lkdtm/perms.c +++ b/drivers/misc/lkdtm/perms.c @@ -31,13 +31,13 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500; * This just returns to the caller. It is designed to be copied into * non-executable memory regions. */ -static void do_nothing(void) +static noinline void do_nothing(void) { return; } /* Must immediately follow do_nothing for size calculuations to work out. */ -static void do_overwritten(void) +static noinline void do_overwritten(void) { pr_info("do_overwritten wasn't overwritten!\n"); return; @@ -110,15 +110,14 @@ void lkdtm_WRITE_RO_AFTER_INIT(void) void lkdtm_WRITE_KERN(void) { - size_t size; - volatile unsigned char *ptr; + unsigned long value_dow = (unsigned long)do_overwritten; + unsigned long value_do = (unsigned long)do_nothing; + size_t size = (size_t)(value_dow > value_do ? + value_dow - value_do : value_do - value_dow); - size = (unsigned long)do_overwritten - (unsigned long)do_nothing; - ptr = (unsigned char *)do_overwritten; - - pr_info("attempting bad %zu byte write at %px\n", size, ptr); - memcpy((void *)ptr, (unsigned char *)do_nothing, size); - flush_icache_range((unsigned long)ptr, (unsigned long)(ptr + size)); + pr_info("attempting bad %zu byte write at %px\n", size, do_overwritten); + memcpy((void *)value_dow, (void *)value_do, size); + flush_icache_range(value_dow, value_dow + (unsigned long)size); pr_err("FAIL: survived bad write\n"); do_overwritten(); -- 2.17.0
